1. Technical Field
This invention relates to a method of producing a catalyzed alkali metal-aluminum hydride.
2. Background Art
Alkali metals (lithium, sodium and potassium) form a wide variety of simple hydrides and complex intermetallic hydrides that are commonly used as reducing agents in various processes of organic chemistry. While simple alkali earth hydrides may be produced by direct reaction between molten alkali metal and hydrogen (at very high pressures and temperatures) preparation of the more complex hydrides of these metals has required development of specialized, individual processes.
Hydrides of aluminum with lithium, sodium, and potassium have been known for many years. A direct synthesis method to produce these materials was first described (French Patent Serial Number 1,235,680). According to Ashby, synthesis of, for instance, NaAlH4 can be performed by placing either the alkali metal or its hydride into an autoclave with activated aluminum powder in a solvent such as tetrahydrofuran. The mixture is subjected to hydrogen at a pressure of 2000 psi (about 135 atm) and heated to 150xc2x0 C. for several hours after which the mixture is cooled, the excess aluminum is separated by filtration, and the NaAlH4 isolated by precipitation using a hydrocarbon additive such as toluene to the tetrahydrofuran solution, followed by vacuum distillation of the tetrahydrofuran. The method is applicable to the production of LiAlH4, NaAlH4, KAlH4 and CsAlH4.
Others (Zakharin, et al., Dokl. Akad. Nauk SSR, vol. 1, No. 145, p. 793, 1962; Dvorak, et al. U.S. Pat. No. 3,357,806; Tranchant, et al. French Patent Serial Numbers 7,020,279 and 6,914,185) developed similar processes each of which relied on the use of an organic solvent.
While alkali-metal based complex hydrides were developed to serve as reducing agents in chemical reactions, other applications of these hydrides have also been considered in recent years. In particular, the development of hydrogen as an alternative to fossil fuels has spurred the search for materials capable of serving as economic sources for hydrogen storage and retrieval. Due to their gravimetric energy densities, hydrides of the alkali metals are very attractive. Most of these hydrides undergo decomposition releasing hydrogen at moderate temperatures ( less than 150xc2x0 C.).
However, the alkali metal hydrides prepared in the traditional manner act only to irreversibly release hydrogen under moderate conditions. While Bogdanovic, et al., (U.S. Pat. No. 6,106,801) have reported that the addition of a transition metal compound acts as a catalyst to aid in the re-absorption of hydrogen, the kinetics of this system have been reported to be slow and unstable. Zaluska, et al., (U.S. Pat. No. 6,251,349) have reported reversible absorption and desorption of hydrogen is achieved in complex alkali metal-aluminum hydride compounds prepared by mechanical mixing/milling mixtures of the simple hydrides without the catalyst reported by Bogdanovic, et al.
The present invention provides a totally different method for preparing alkali metal-aluminum hydrides which is based on simple a two step-process. The resulting hydrides exhibit outstanding reversible hydrogenation properties.
In accordance with one aspect of the invention there is provided a method of producing an alkali metal-aluminum hydride comprising mechanically milling powders of a simple alkali metal hydride material with a metal and a titanium catalyst compound followed by high pressure hydrogenation at temperatures above about 60xc2x0 C. The alkali metal hydride is NaH, the metal powder is aluminum, and the titanium catalyst compound is TiCl3, TiF3, or a mixture of equal parts of these two compounds.
In another aspect of this invention, there is provided a method of producing an alkali metal-aluminum hydride comprising mechanically milling powders of an alkali metal with a metal powder and a titanium catalyst compound, wherein the alkali metal is sodium, and the metal powder is aluminum and the titanium catalyst compound is TiCl3, TiF3, or a mixture of equal parts of these two compounds.
In yet another aspect of this invention, there is provided a method for preparing an alkali metal-aluminum hydride in a two-step solid-state reaction, wherein the first step comprises mechanical milling and the second comprises high pressure hydrogenation at elevated temperatures.
In the first step, the method is performed with dry powders of the components (i.e., without a solvent or any other suspension aid) under a blanket of a dry inert gas such as argon. The method is accomplished by subjecting the chosen reagent materials to a mechanical milling means, wherein the milling means consists of a ball mill, a plate or impact grinder, a blade, rod or whisk mixer, blender, or agitator.
In the second step, the method is completed by heating the milled contents to a temperature of about 100xc2x0 C., while maintaining a hydrogen gas pressure in the container above the equilibrium plateau pressure of the reaction, (above about 30 atm hydrogen).
In yet another aspect of the invention the pressure of hydrogen gas is maintained at about 100 atm of hydrogen while heating the milled mixture to an initial temperature of about 125xc2x0 C.
In another aspect of the invention there is provided catalyst doped alkali metal-aluminum hydrides which effectively function as a recyclable source/sink for hydrogen gas.